The broad gain bandwidth of conventional fiber-laser systems allows for operation over a wide range of wavelengths, or even tunable operation. For the simplest fiber-laser system with cavity mirrors having reflectivity across a broad range of wavelengths, the output wavelength can be very broad and can vary with pump power, fiber length, and/or other parameters. The power that can be generated from fiber lasers and fiber-laser amplifiers can often be limited by nonlinear optical effects in the gain and/or delivery fibers used in the system.
In order to generate single beams of laser light with very high power levels, it is desirable to do spectral-beam combining (SBC) of a plurality of laser beams, such as described in U.S. Pat. No. 7,386,211 titled “METHOD AND APPARATUS FOR SPECTRAL-BEAM COMBINING OF MEGAWATT-PEAK-POWER BEAMS FROM PHOTONIC-CRYSTAL RODS” and U.S. Pat. No. 7,199,924 titled “APPARATUS AND METHOD FOR SPECTRAL-BEAM COMBINING OF HIGH-POWER FIBER LASERS,” each of which is hereby incorporated by reference in its entirety. It is desirable to produce high peak and average powers from the fiber lasers and amplifiers used in SBC systems. Stimulated Brillouin scattering (SBS) and other nonlinear effects such as self-phase modulation (SPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS) are the main effects limiting the output power and pulse energy of a fiber amplifier or laser. To suppress these effects in a fiber amplifier/laser, it is desirable to use a rare-earth-doped (RE-doped) double-clad fiber with a large core. The large core provides two benefits: spreading the light over a larger core decreases the intensity driving the nonlinear processes, and increasing the core/cladding diameter ratio increases pump absorption, enabling the shortening of the fiber to further reduce nonlinearities. When good beam quality is required, however, increasing the core diameter of the fiber requires that the fiber numerical aperture (NA) be decreased, in order that higher-order modes cannot propagate in the fiber. Using relatively large-core, low-NA fibers with mode-filtering techniques has been demonstrated to achieve good beam quality, but there are practical disadvantages to the use of such fibers. Fibers with very low values of NA exhibit large bending losses, even for relatively large-radius bends. With fibers having the lowest NA, the fiber must be kept quite straight, otherwise the optical amplifier and/or laser has very low efficiency as the bending loss becomes too high. Since a typical laser oscillator or amplifier might require on the order of a meter or more of gain fiber, the inability to coil the fiber has precluded compact packaging of the fiber-laser system. Stimulated Brillouin Scattering (SBS) is a well-known phenomenon that can lead to power limitations or even the destruction of a high-power fiber-laser system due to sporadic or unstable feedback, self-lasing, pulse compression and/or signal amplification.
U.S. Pat. No. 6,192,062, issued Feb. 20, 2001, to Sanchez-Rubio et al. titled “BEAM COMBINING OF DIODE LASER ARRAY ELEMENTS FOR HIGH BRIGHTNESS AND POWER” and U.S. Pat. No. 6,208,679, issued Mar. 27, 2001, to Sanchez-Rubio et al. titled “HIGH-POWER MULTI-WAVELENGTH EXTERNAL CAVITY LASER” describe the fundamental techniques of spectral beam combining, and both are incorporated herein by reference.
In some embodiments, the gratings used for spectral-beam combining are “blazed,” i.e., formed with V-grooves having sidewall angles that are asymmetrical with respect to a vector normal to the overall surface of the grating. U.S. Pat. No. 3,728,117, issued Apr. 17, 1973, to Heidenhain et al., titled “OPTICAL DIFFRACTION GRID” (incorporated herein by reference), describes one method for making blazed gratings having asymmetric grooves. U.S. Pat. No. 4,895,790, issued Jan. 23, 1990, to Swanson et al., titled “HIGH-EFFICIENCY, MULTILEVEL, DIFFRACTIVE OPTICAL ELEMENTS” (incorporated herein by reference), describes a method for making blazed gratings having asymmetric grooves using binary photolithography to create stepped profiles. U.S. Pat. No. 6,097,863, issued Aug. 1, 2000, to Chowdhury, titled “DIFFRACTION GRATING WITH REDUCED POLARIZATION SENSITIVITY” (incorporated herein by reference), describes a reflective diffraction grating with reduced polarization sensitivity for dispersing the signals. The Chowdhury grating includes facets that are oriented for reducing efficiency variations within a transmission bandwidth and that are shaped for reducing differences between the diffraction efficiencies in two orthogonal directions of differentiation. U.S. Pat. No. 4,313,648 issued Feb. 2, 1982, to Yano et al., titled “PATTERNED MULTI-LAYER STRUCTURE AND MANUFACTURING METHOD” (incorporated herein by reference) describes a manufacturing method for a patterned (striped) multi-layer article.
U.S. Pat. No. 6,822,796 issued Nov. 23, 2004, to Takada et al., titled “DIFFRACTIVE OPTICAL ELEMENT” (incorporated herein by reference), describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859, issued Oct. 25, 2005, to Hoose et al., titled “GRATING DEVICE WITH HIGH DIFFRACTION EFFICIENCY” (incorporated herein by reference), describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 titled “MULTILAYER DIELECTRIC DIFFRACTION GRATINGS” was issued May 25, 1999, to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 6,212,310 titled “HIGH POWER FIBER GAIN MEDIA SYSTEM ACHIEVED THROUGH POWER SCALING VIA MULTIPLEXING” was issued Apr. 3, 2001, to Waarts et al., and is incorporated herein by reference. This patent describes certain methods of power scaling by multiplexing multiple fiber gain sources with different wavelengths, pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber. One method described by Waarts et al. is similar to that shown in the present invention shown in FIG. 2A1, described below, where a plurality of input laser beams of differing wavelengths are directed at different angles to a diffraction grating, which diffracts the beams into a single output beam, however, this output beam necessarily has a wavelength linewidth-dependent chromatic divergence introduced by the grating. The present invention includes many distinguishing features not in Waarts et al.
There is a need for spectral-beam-combining (SBC) laser systems, particularly those employing fiber lasers and/or fiber optical amplifiers, wherein the output of each one of a plurality of beams is measured and, based on the measurements, feedback is provided to control the fiber lasers and/or fiber optical amplifiers.